Vacuum brake boosters are used to amplify the braking force exerted on the foot brake pedal of a motor vehicle. Due to their simple and economical construction, they are the most widely used type of booster in motor vehicles.
FIG. 1 shows a schematic representation of a conventional vacuum brake booster (VBB) known from the prior art. The VBB essentially includes a working chamber 2, a vacuum chamber 1 having a vacuum connection 3 and a membrane 7 that divides the two chambers 1, 2 from one another. At vacuum connection 3, a vacuum source (not shown) is connected that is for example driven by the internal combustion engine and that produces a predetermined vacuum in vacuum chamber 1. In the central area of VBB 8, a double valve 4 is situated that performs two functions, namely a) dividing working chamber 2 from vacuum chamber 1, or connecting the two chambers 1, 2 to one another, and b) ventilating working chamber 2, or separating it from the air of the surrounding environment.
In the unbraked state, the connection between vacuum chamber 1 and working chamber 2 is open. Thus, the same vacuum prevails in both chambers 1, 2. When there is an actuation of the foot brake pedal, the two chambers 1, 2 are separated from one another, and working chamber 2 is ventilated. Dependent on braking force F exerted via piston rod 6, a pressure level arises that is between the vacuum in vacuum chamber 1 and the environmental pressure. The force resulting from the pressure difference acting on working membrane 7 amplifies the braking force exerted on the brake pedal. The auxiliary force portion produced by VBB 8 is essentially dependent on the constructive design of VBB 8 and on the vacuum prevailing in vacuum chamber 1. After the release of the brake pedal, the ventilation with environmental air is interrupted, and the chamber valve is again open. In this way, both chambers 1, 2 are charged with a vacuum from the vacuum source.
FIG. 2 shows a typical transmission characteristic of a VBB 8, showing the brake pressure p (pre-pressure) acting in the brake system over the force F exerted on the foot brake pedal. VBB 8 begins operation starting from a predetermined minimum force F0 that is required for the actuation of the mechanical components, and then amplifies brake pressure p in linear fashion with increasing braking force F. Here, the gain factor is designated k. In the linear area of the characteristic curve, the auxiliary force portion increases constantly up to a saturation point 11 (AP). At saturation point 11, the maximum pressure difference between working chamber 2 and vacuum chamber 1 is reached. Environmental air pressure then prevails in working chamber 2. If the braking force F on the foot brake pedal is increased further, brake pressure p increases only without amplification.
Conventional VBBs 8 are standardly constructed in such a way that the saturation point 11 is not exceeded, or is not significantly exceeded, even given maximum actuation of the foot brake pedal. However, in the case of VBBs 8 that are dimensioned too small, or if there is an insufficient vacuum supply in vacuum chamber 1, saturation point 11′ lies below blocking pressure level 14. In this case, braking force F continues to be linearly amplified only up to saturation point AP′, and is subsequently transmitted only in unamplified fashion (line 13). This has the result that after saturation point AP′ has been exceeded, a further increase in the braking force requires a significantly increased exertion of force on the brake pedal.
In order to remove this problem, it is known to switch over to a hydraulic booster system when saturation point 11 has been reached, and to activate the hydraulic modulator of a vehicle dynamics regulation system (e.g. ESP) in order to build up additional brake pressure. For this, it is necessary to recognize the saturation point precisely and to switch over to hydraulic boosting at the right point in time.
From the prior art, it is known to measure the pressure difference between working chamber 2 and vacuum chamber 1 using a pressure sensor 9b situated in working chamber 2 and a pressure sensor 9a situated in vacuum chamber 1, and to switch over to hydraulic boosting when a maximum pressure difference has been achieved. However, this requires two pressure sensors, which is relatively expensive.